The only immediate fixes the spec need is to have Mass Regeneration chain up to 40-50 yards and then to fix the bugs with the Regeneration spells not consuming either the AB debuff or Clearcasting buff.
Otherwise I promise you most people are just playing the build incorrectly. Even with the proposed buff, Mass Regeneration is a niche spell that you want to be using when you will be healing at least three people. Ideally when they will benefit from both the direct heal and follow-Beacon healing. It’s not a primary healing method. Arcane is also not a spot healer. It is a tank healer with the ability to counter significant party-wide damage. Your healing partners need to be defaulted to raid/spot healing and assisting you with tank healing as needed, while you do the reverse.
Also yes, the build is highly dependent on being proactive instead of reactive. It is the most difficult healer to play perfectly, but not because the core design is flawed. The build is actually absurdly broken in its current iteration; it’s just not broken in a way where it has no weaknesses and is braindead to pilot.
And just to be clear, I enjoy the build so much that I’m about to hit 25 on my second mage because I wanted to swap servers and fell in love with the spec after swapping to it around 20-21 on my first mage. I am talking from the perspective of as much of a main as you can be of the build right now.